It is valid for four years and then will be renewed with a five year certificate in 2013. Kevin Bocek, vice president of security company Venafi, will later say, “Most security professionals wouldn’t recommend that. Google uses three-month certificates.” The certificate used a standard strength 2,048-byte encryption key. However, it doesn’t use “perfect forward secrecy.” That means that if the key is broken, multiple emails can be accessed. (ComputerWorld, 3/11/2015)

A 2016 FBI report will confirm this, mentioning that the certificate is valid until September 13, 2013, at which time a new certificate is obtained which is valid until September 13, 2018. (Federal Bureau of Investigation, 9/2/2016)